I'm thinking a PCI-E x1 or x4 interface would be nice. Shouldn't be impossible to implement on a daughtercard. Opens up myriad possibilities for expansion and interconnect.
I don't think we all realize just what mikej has on his hands here. The idea now that we can have a compact computer that can mimic nearly any computer that preceded it, and indeed enable the resurrection of such platforms as viable consumer products, even for novelty's sake, is simply amazing. For sure he isn't the first to come up with the idea, but the implementation of this board configuration with its expansion bus and virtually all needed connectors to coexist in the current device arena is just what such a product needs to become something people want for Christmas - especially budget-minded nostalgia-addled middle aged men like myself

CUSA can offer a cheap PC-based emulator stuffed in an impressively made case, but he can offer a single-board compact solution that behaves faithfully like any old PC (generic term implied) in a box smaller than a mac mini.
I know this will raise some hackles, but I personally would love to run Android on it. If it'll run Linux it'll happen. It has an ARM core. Why not?